Search PubMed⌕ Search

Biomedical subjects

A R Means

Publications and source records attributed to A R Means.

At least 235 records · Page 13Linked to original sources
PubMed1977

Ca2+-dependent regulation of cyclic-AMP phosphodiesterase by parvalbumin.

Carp parvalbumin has been shown to activate rat brain phosphodiesterase in a Ca2+-dependent manner. The concentration of Ca2+ required for half-maximal stimulation is 1.4 X 10(-7) M, whereas rat testis Ca2+-dependent regulator (CDR) of phosphodiesterase required 1.2 X 10(-6) M Ca2+. The difference in the slopes of the two curves demonstrated that the activation induced by parvalbumin was not the result of a small contamination by CDR. In addition, it has been shown that Ca2+ binding to parvalbumin parallels its activation of phosphodiesterase. These data suggest that Ca2+ must bind to a single specific metal binding site before phosphodiesterase can be fully activated.

3',5'-Cyclic-AMP Phosphodiesterases↗
PubMed1977

Biological cross-reactivity of rat testis phosphodiesterase activator protein and rabbit skeletal muscle troponin-C.

Phosphodiesterase activator protein and troponin-C have been purified from rat testis and rabbit skeletal muscle, respectively. The two proteins appear to be structurally distinct since the activator protein migrates faster than troponin-C on sodium dodecyl sulfate-polyacrylamide gels. Each of the calcium-binding proteins will, however, substitute for the other in their respective biological systems. Testis activator protein forms a complex with rabbit muscle troponin subunits TnI and TnT soluble in low salt. This hybrid complex (AIT) can regulate rabbit skeletal muscle actomyosin ATPase activity. AIT regulation, although influenced by free Aa2+ levels, is distinct from that of native troponin. Likewise, muscle troponin-C can substitute for activator protein in the stimulation of cyclic nucleotide phosphodiesterase. Troponin-C will fully stimulate phosphodiesterase although its affinity is 600-fold lower than that of activator protein. Ca2+ regulation studies demonstrate that both proteins require micormolar levels of free Ca2+ to induce phosphodiesterase activation. Activator protein requires 1.2 x 10(6) M and troponin-C, 1.9 X 10(6) M free Ca2+ for half-maximal stimulation of phosphodiesterase. The biological cross-reactivity of these proteins supports the sequence homology recently reported by Watterson et al. (Watterson, D.M., Harrelson, W.G., Keller, P.M., Sharief, F., and Vanaman, T.C. (1976) J.Biol. Chem. 251, 4501-4513). In addition, this preliminary study suggests that this nonmuscle troponin-C-like protein potentially may function in other Ca2+-regulated cellular events in addition to its moculation of cyclic nucleotide levels.

Adenosine Triphosphatases↗
PubMed1977

Female steroid hormones and lipoprotein synthesis in the cockerel: effects of progesterone and nafoxidine on the estrogenic stimulation of very low density lipoproteins (VLDL) synthesis.

Since progesterone is known to modulate the effects of estradiol in a number of organ systems, we have studied the effects of the two hormones on very low density lipoproteins (VLDL) synthesis in the cockerel. We have also examined the effect of an estrogen antagonist, nafoxidine-HCl, on the process. When three week-old cockerels were treated with a single injection of estradiol (1 mg) or estradiol (1 mg) + progesterone (2 mg), plasma VLDL increased within 5 h, reached a peak at 48 h and returned to baseline values at 68 h. There was no significant difference between estradiol or estradiol + progesterone treated animals. Liver slices were prepared from similarly treated animals and incubated in vitro with L-[3H]lysine for 2 h. Following homogenization and centrifugation at 105,000 X g, newly synthesized VLDL were quantitiated in the supernatant fluid by immunoprecipitation with a monospecific antibody. Radioactivity incorporated into VLDL was found to increase from low baseline levels to a peak at about 17 h after treatment. There was, again, no significant difference between estradiol and estradiol + progesterone-treated animals. Estrogen priming or progesterone pretreatment also did not significantly alter the VLDL biosynthetic response in liver slices to estradiol alone or in combination with progesterone. When the estrogen-antagonist nafoxidine-HCl (5 mg) was administered simultaneously with estradiol (1 mg), it totally inhibited the VLDL biosynthetic response in liver slices in vitro. The interaction of estradio, progesterone and nafoxidine-HCl on the hepatic synthesis of VLDL should serve as a valuable model for the study of VLDL synthesis and its regulation as well as the mode of action of the sex steroid hormones in the liver.

Animals↗
PubMed1977

Stability of nuclear triiodothyronine binding sites.

Rat liver nuclei have recently been shown to possess limited, high affinity binding sites for triiodothyronine, only a fraction of which are believed to be filled in vivo. We have found that preincubation of nuclei in the absence of added triiodothyronine leads to a rapid decrease in binding capacity. This loss of binding sites is considerably faster at 37 C than at 4 C. On the other hand, once binding equilibrium is reached between nuclear bound and free triiodothyronine the level of binding does not change during the time in which a considerable loss of binding had occurred when nuclei were preincubated in the absence of hormone. Also the rate of dissociation of triiodothyronine from nuclei is slower than the rate of loss of binding sites during preincubation in the absence of added hormone. Thus, it appears as if unfilled sites are being lost during the preincubation period. Administration of cycloheximide or actinomycin D, inhibitors of protein and RNA synthesis, respectively, to rats caused a decrease in the amount of triiodothyronine which could be bound in the in vitro binding assay to liver nuclei.

Animals↗
PubMed1977

Estrogen induction of plasma vitellogenin in the cockerel: studies with a phosvitin antibody.

The effects of estrogen on plasma vitellogenin have been studied in the cockerel by immunoprecipitation techniques using an antiserum prepared against the egg yolk phosphoprotein, phosvitin. The antiserum gave precipitin lines of complete identity to phosvitin and to vitellogenin which was isolated from hen plasma by DEAE-cellulose chromatography and by affinity chromatography using anti-phosvitin coupled to Sepharose 4B. The cross-reactivity of vitellogenin and phosvitin adds support to the concept that plasma vitellogenin is the precursor phosphoprotein of egg yolk phosvitin. In the three-week old cockerel, anti-phosvitin produced no detectable immunoprecipitate in the plasma. However, after a single sc injection of diethylstilbestrol (2.5 mg), plasma vitellogenin levels began to increase at 4 h and reached a maximum 20-30 h after hormone administration. The increase in plasma levels of triglyceride paralleled those of vitellogenin. These studies suggest that there is no significant time lag in the estrogenic induction of plasma vitellogenesis in the cockerel, the longer lag periods observed by other investigators may be a function of the sensitivity of the assays used for detecting vitellogenin.

Amino Acids↗
PubMed1977

Characterization and follicle stimulating hormone activation of Sertoli cell cyclic AMP-dependent protein kinases.

The Sertoli cell of the rat testis contains two cytoplasmic forms of adenosine 3',5' monophosphate (cyclic AMP)-dependent protein kinase, designated as Peak I and Peak II, which change in relative proportion during Sertoli cell maturation. Peak I and Peak II differ in their subunit interaction. Thus, while the substrates, ATP and histone, affect cyclic AMP binding to Peak I, neither of these compounds affect the binding of cyclic AMP to the Peak II enzyme. The effects of cyclic AMP binding to Peak I appear to be due to the fact that histone and high ionic strength cause dissociation of the Peak I holoenzyme, whereas ATP stabilizes the holoenzyme complex against dissociation. The Peak II holoenzyme is not affected by either salt of histone and is only dissociated by cyclic AMP. Once dissociated, the subunits of Peak II will rapidly reassociate under low salt conditions whereas the subunits of Peak I will not reassociate. By utilizing the distinct properties of Peak I and Peak II, it is possible to demonstrate the activation of both Peak I and Peak II Sertoli cell protein kinase in response to FSH.

Animals↗